1. Field of the Invention
The present invention relates to an optical amplifier enabling the large capacity and the long distance of an optical communication system, and in particular, to an optical amplifier in which a population inversion state is formed by a pumping light supplied to an optical amplification medium and an ASE light generated in the optical amplification medium.
2. Description of the Related Art
In recent years, demands of information have been remarkably increased with the development of Internet technology. In a trunk optical transmission system to which information capacity is collected, the larger capacity, the flexible network formation and the like have been demanded. A wavelength division multiplexing (WDM) transmission system is the most effective means at present stage, as the technology for coping with such a system demand, and the commercialization thereof is now progressed.
For realizing the above wavelength division multiplexing transmission system, an optical amplifier which amplifies optical signals using an optical fiber doped with a rare-earth element, is one of key components, since this optical amplifier can collectively amplifies a wavelength division multiplexed signal light utilizing a wide gain band thereof. As such an optical amplifier, an erbium (Er)-doped fiber optical amplifier (EDFA) is typical.
In recent years, it has been known that the EDFA not only has an amplification band corresponding to 1530 to 1565 nm wavelength band, so-called C-band (conventional wavelength band), which has been mainstream, but also contains 1570 to 1605 nm wavelength band, so-called L band (long wavelength band) as the amplification band thereof. Therefore, in the existing EDFA system, it is possible to amplify a WDM signal light in which optical signals of about 200 waves are arranged in a band combining the C-band and the L-band (refer to Japanese Unexamined Patent Publication No. 2000-77755, Japanese National Phase Patent Publication No. 2002-528901 and the literature “Review of wideband gain flattening EDFA for L-Band amplification”, Sawada et al., Mitsubishi Cable Industries Review, Vol. 96, pp. 45-48).
FIG. 7 is a block diagram showing a basic configuration of a conventional EDFA.
The conventional EDFA shown in FIG. 7 comprises: an erbium-doped fiber (EDF) 101; a pumping semiconductor laser (LD) 102 and a WDM coupler 103 which supply a pumping light Lp to the EDF 101; and optical isolators 104 and 105 arranged on both end portions between an input terminal IN and an output terminal OUT. In this configuration example, a forward pumping configuration is applied, so that the pumping light Lp is supplied from one end of the input side of the EDF 101, and a signal light Ls and the pumping light Lp are propagated to the same direction, thereby obtaining a good noise characteristic (NF). Further, in the conventional EDFA, there are frequently applied a configuration for feedback controlling a drive current to be given to the pumping semiconductor laser 102 based on a monitoring result in an input and output power monitor (not shown in the figure, here), so that an output power level or a gain becomes constant.
It has been known that, in the case where a signal light in the L-band is amplified by the conventional EDFA having the above basic configuration, an optical amplification operation of the EDFA shows a frequency response characteristic which is changed in two-steps, as shown in FIG. 8 and FIG. 9 for example. This frequency response characteristic of two-steps occurs caused by the absorption process of pumping light for when the EDF 101 is brought into a population inversion state.
Namely, the EDF 101 is brought into the population inversion state by the process in which the pumping light Lp supplied from the pumping light source 102 to the EDF 101 via the WDM coupler 103 is directly absorbed, and the process in which an amplified spontaneous emission (ASE) light generated in the EDF 101 is reabsorbed as the pumping light. Since a delay occurs in an optical amplification response characteristic obtained based on the re-absorption of the ASE light, compared with an optical amplification response characteristic obtained based on the absorption of the pumping light Lp, an optical amplification response characteristic in the entire EDF 101 is changed in two-steps. FIG. 8 shows one example specifically representing this optical amplification response characteristic in which a modulation frequency of pumping LD is in the horizontal axis and a phase is in the vertical axis. A phase characteristic in FIG. 8 represents, as a phase difference, a time difference between timing at which the power of the signal light Ls is changed and the time when the power of the pumping light Lp is changed, based on the fact that, when the pumping light Lp is modulated with a sine wave or the like to be supplied to the EDF 101, the signal light Ls is amplified in response to a modulation component of the pumping light Lp. Then, FIG. 9 shows a gain characteristic which is converted from the phase characteristic in FIG. 8.
The optical amplification showing the response characteristic of two-steps as described above has a possibility to cause a control error, since a phase deviation occurs in the feedback control of the drive current to be given to the pumping semiconductor laser 102. Therefore, the conventional L-band EDFA has a problem in that a so-called first-order lag feedback control is hard to be performed.
The present invention has been accomplished in view of the above problems and has an object to provide an optical amplifier capable of realizing a good response characteristic in a wide frequency band, even when a population inversion state is formed by a pumping light supplied to an optical amplification medium and an ASE light generated in the optical amplification medium.